The following invention relates to a voltage booster device of the type used in non-volatile memories.
As is known, for non-volatile memories such as, in particular, multilevel Flash memories, it is necessary to have a higher voltage than the supply voltage commonly used not only in programming and deleting operations, but also for the operations of reading the content of the memory cells.
Therefore, unless one provides for the required voltages to be supplied from outside the integrated circuit on which the memory is created, they must be generated inside the integrated circuit itself; this is typically the case of a Single Power Supply circuit.
This calls for the need to use voltage booster devices, created in the same integrated circuit.
The voltage booster devices commonly used include boosters based on the charge pump principle. As is known, a charge pump booster behaves as a voltage generator with non-null output resistance, for which the current that can be requested of it is inversely proportionate to the voltage that it must supply.
Moreover, this type of booster does not make it possible to supply voltage with the necessary precision and, therefore, an adjustment circuit of the booster output voltage is conveniently used.
With the aim of reducing consumption, when operations are not being performed on the non-volatile memory the voltage booster device is deactivated or rather, it is taken to a wait or stand-by status.
To this ends, the document EP-A-1113450 describes a voltage booster device including, in addition to booster and a regulator, also a stand-by booster device to operate on the stand-by input. At the stand-by input, an output node of the booster and an output node of the voltage regulator are connected to one another and with an output terminal of the stand-by voltage booster device. This document describes that the stand-by booster device is used to supply to such nodes a voltage necessary to compensate the discharge due to junction leakage currents (inverse saturation currents) of the transistors connected to it. According to the abovementioned document, thanks to the action of this stand-by booster device it is possible, on exiting stand-by, to reach the reading voltage required quickly and accurately.
With particular reference to a voltage booster device used to supply the reading voltage of a cell of the type described in the document EP-A-1113450, phenomena of saturation of the voltage value of the output node of this device such as to vanquish the improvement action of the stand-by booster device or, even, such as to be able to cause an erroneous reading of cell content have been observed. Moreover, for this type of conventional device a strong dependence of its performance on the piloting signal times taken to bring the device itself to stand-by and in the active state has been detected.
One aim of the present invention is to provide a voltage booster device that presents superior performances than those of the conventional devices described above.
The aim of the present invention is achieved by a voltage booster device as defined in the annexed claims 1 to 21.
Also object of the present invention is a memory system including a voltage booster device as defined in the annexed claim 22.